An envelope is the glass portion of a cathode ray tube (CRT), and generally consists of three basic glass sections: a relatively flat face-plate, through which an image is viewed, a conical funnel section to which the faceplate is sealed, and a neck section comprising a length of tubing sealed to the vertex of the funnel and in which the electronic assembly (termed the "gun") for generating the image is sealed. The area where the neck section meets the conical funnel section is commonly known as the yoke, and the angle at which the conical funnel flares out therefrom is known as the yoke angle of the funnel.
During the manufacture of CRTs, after a face-plate has been sealed to the outspread end of the funnel and the neck has been sealed to the constricted end of the funnel to form a complete envelope, the electronic gun is inserted into the neck and sealed thereto. This operation frequently leads to rejects resulting from such defects as cracking, chipping, misalignment, and poor seals. Because of the relatively high costs of envelopes, the CRT manufacturer, in an effort to reduce costs, will commonly remove the seal between the face-plate and the funnel, crack off the portion of the neck having the electronic assembly therewithin, and either repair the funnel in-house or returns the funnel with the remainder of the attached neck to the glass component manufacturer to be repaired. The repair activity removes the old neck and attaches and seals a new replacement neck thereon.
The dimensions and contours of the repaired funnels must continue to satisfy prescribed manufacturer specifications, and thus it is important that the old neck be cracked off and the new neck be attached with precision. This is particularly important for straight ended necks (i.e., cylindrical, as opposed to those having a flared distal end). With flared necks, the electronic gun is typically sealed well inside the distal end of the neck, after which the flared distal end is removed. With straight ended necks, however, the electronic gun is sealed at the very end of the neck. Consequently, when attaching straight ended replacement necks, it is even more important that the distal end of the neck be located accurately.
To further complicate matters, the component manufacturer commonly receives a variety of different size glass funnels, having different sized viewing screens and/or yoke angles, and which have been fabricated by different glass manufacturers and/or at different time periods. As a result, the funnels to be repaired will commonly be in a random sequence of different sizes, all of which require the existing neck to be cracked off and a new neck attached. These circumstances self-evidently make consistent dimensional control extremely difficult and time consuming.
The cross-sectional area of the funnel across which the neck is to be cracked off is commonly known as the crackoff plane. To meet the stringent yoke-to-end-of-neck dimensions required by manufacturers, it is critically important that, before attaching the necks, an extremely precise crackoff plane be defined. Because of the random shape and size encountered from funnel to funnel, it is difficult to consistently obtain such a precise crackoff plane location. In addition, after the crackoff procedure, the replacement neck must be sealed to the funnel at the plane of glass crackoff while still complying with these critical yoke-to-end-of-neck dimensional tolerances.
It is therefore desirable, during these crackoff and resealing operations, to have a consistent gauging point from which to measure and locate the crackoff plane and end of neck location. One such consistent reference point commonly used in the CRT industry is known as the yoke reference line (YRL). The YRL is a manufacturer's specification which exists for any model and size CRT, and for each model and size funnel does not vary. The yoke reference line is a particular diameter which is usually located in the yoke area of the funnel. Currently, to determine the location of the YRL, a donut shaped gauge is manually placed over the neck of the funnel and slid down onto the yoke section until it contacts the desired diameter. Unfortunately, such manual gauging systems lack consistency and accuracy.
Furthermore, after the YRL is located, the crackoff plane location is typically located manually by measuring a predetermined distance from the YRL. This second manual measurement adds further margin for error and inconsistency into the neck resealing process.
After the crackoff plane has been located, the funnel is moved to a scoring apparatus to score the old neck at the crackoff plane. This is typically done by bringing a rotating glass scoring device into contact with the inside diameter of the funnel, and scoring the funnel at the predetermined crackoff location. The location of the scoring device is again done manually.
After scoring the glass, the scoring device is withdrawn from the funnel and the funnel is manually placed near a burner system so that the flames of the burner contact the funnel at the scored crackoff location. These flames cause the glass to break from the scored surface completely through the thickness of the glass. The unwanted glass (commonly known as "moil") then is typically allowed to fall away by gravity.
A replacement neck must then be sealed to the cracked off funnel. This is typically achieved by manually aligning the funnel with a replacement neck and then manually controlling a suitable mechanical apparatus to bring the neck in contact with the crackoff surface of the funnel, while a flame from a burner heats the joining area of the two components. Once the sealing process has been completed, the funnel is removed and allowed to cool. The funnel is then commonly annealed to complete repair of the funnel.
As is evident from the above description, the current practice is time consuming, and the manual gauging, measuring, and apparatus control methods are relatively inconsistent and inaccurate and require a skilled operator to be successfully completed.
Thus, the need remains for a more efficient method and apparatus for carrying out the process of cracking off the necks of damaged funnels at a precisely calculated point, and thereafter sealing a replacement neck to the funnel at the crackoff plane. There is a particular need for such a method and device which are capable of quickly and easily performing these operations on a variety of different funnel designs, having different sized viewing screens and different shaped yoke angles. Because of the intermittent nature of such repairs, it is also desirable that the process be operable using relatively unskilled labor.